An in-trail procedure (ITP) is a protocol followed by an aircraft that desires to change its current flight level to a new flight level by descending or climbing in front of or behind one or more potentially blocking aircraft flying at an intervening flight level. In accordance with ITP criteria, certain conditions must be satisfied before the flight crew member issues a request for clearance to proceed with the flight level change. Whether or not the conditions are satisfied will depend on a number of dynamically changing factors associated with the host aircraft and other aircraft, such as the current geographic position of the aircraft, the current speed of the aircraft, the current heading of the aircraft, the desired new flight level, and the current flight level.
Modern flight deck instrumentation might include a flight-management system display that provides a two-dimensional representation of a host aircraft and neighboring aircraft. Such display systems typically provide a number of parameters and visual indicators that enable a pilot to form a quick mental picture of the vertical situation of the host aircraft. For example, such a system might include displays of an aircraft symbol, the aircraft altitude, the vertical flight plan, and terrain. In this manner, a member of the aircraft flight crew can obtain information related to the vertical situation of the aircraft relative to other aircraft with a simple glance at the display system.
Such a system could be used to identify the vertical position of potentially blocking aircraft for purposes of an ITP; however, a flight crew member may still need to mentally interpret the traffic situation and/or perform calculations related to the designation of potentially blocking aircraft and related to the determination of whether conditions merit that an ITP protocol be used for a desired flight level change. This is further complicated by the many factors that inference the perceived benefit of an ITP such as wind profile at various flight levels, traffic, fuel performance, etc.